Method and apparatus for controlling a turbogenerator system

ABSTRACT

An apparatus for controlling a turbogenerator system when a power electronics circuit of the turbogenerator system is unable to provide a sufficient load on the turbogenerator to prevent the turbogenerator from accelerating uncontrollably is described. The apparatus comprises a monitor including a first sensing device operable to detect a condition of the turbogenerator, and a brake controller responsive to a turbogenerator detection output from the first sensing device to issue a first brake control signal for operating a brake circuit to prevent the turbogenerator from accelerating uncontrollably and a second brake control signal for operating the brake circuit to permit resumption of normal operation of the turbogenerator.

The present invention concerns a method and apparatus for controlling aturbogenerator system. Some embodiments of the present invention relatemore particularly to controlling a turbogenerator system duringdisturbances of the local electrical grid network.

In normal operation, the speed of the turbogenerator in such a system iscontrolled by exporting power to the local electricity grid. If theturbogenerator speed begins to increase then the output power isincreased to slow the turbogenerator down. By this means, theturbogenerator is held at a constant speed. However, in the event of agrid disturbance or outage, the turbogenerator would accelerateuncontrollably, in the absence of any additional controls.

As is well known, the electrical grid is affected by disturbances, whichmay last from a few micro-seconds to periods of downtime or outagelasting for seconds, minutes or even hours. Most countries have nationalor state regulations, which require generators to disconnect from theelectrical grid if the grid voltage or frequency falls outside certainlimits for more than a predetermined period, typically a few hundredmilli-seconds, depending on the utility that is receiving the outputpower. For shorter disturbances or outages, up to the time limit allowedby the utility, it is important for the generator to continue workingduring the disturbance.

The present invention in its preferred form as described below seeks toaddress such problem by enabling a turbogenerator system to continue tooperate during disturbances of the local electrical grid for an amountof time that may be sufficient to ride through these relatively shortgrid disturbances.

More particularly, the invention in its preferred form provides a meansfor controlling a turbogenerator during a period when the electricalgrid is affected by disturbances.

However, the invention is also applicable more generally in othercircumstances when the speed of the turbogenerator cannot be controlledby exporting power. One such circumstance might be that theturbogenerator temporarily generates more power than a power electronicscircuit is able to export. Another such circumstance might be that thepower electronics circuit develops a temporary fault or overheats and istherefore temporarily unable to export power. Yet another suchcircumstance might be where the turbogenerator is being used to power alocal load (for example an electrical machine, heater, battery chargeror localised power distribution system), and that local load is unableto provide a sufficient load on the power electronics circuit todissipate the power generated by the turbogenerator.

According to an aspect of the invention, there is provided an apparatusfor controlling a turbogenerator system when a power electronics circuitof the turbogenerator system is unable to provide a sufficient load onthe turbogenerator to prevent the turbogenerator from acceleratinguncontrollably, comprising: a monitor including a first sensing deviceoperable to detect a condition of the turbogenerator, and a brakecontroller responsive to a turbogenerator detection output from thefirst sensing device to issue a first brake control signal for operatinga brake circuit to prevent the turbogenerator from acceleratinguncontrollably and a second brake control signal for operating the brakecircuit to permit resumption of normal operation of the turbogenerator.Preferably, the power electronics circuit is deactivated during a griddisturbance, and the apparatus comprises a second sensing deviceoperable to detect a condition of the grid, and a trigger circuitresponsive to a grid detection output from the second sensing deviceindicating a cessation of the grid disturbance to reactivate the powerelectronics unit and begin exporting power to the grid.

According to one embodiment, apparatus for controlling a turbogeneratorsystem during a grid disturbance comprises: a monitor including a firstsensing device operable in response to a grid disturbance to detect acondition of the turbogenerator and a second sensing device operable inresponse to a grid disturbance to detect a condition of the grid, abrake controller responsive to an output from the first sensing deviceto issue a first brake control signal for switching on a brake toprevent the turbogenerator from accelerating uncontrollably and a secondbrake control signal for switching off the brake to permit resumption ofnormal operation of the turbogenerator, and a trigger circuit responsiveto an output from the second sensing device indicating a cessation ofthe grid disturbance to activate a convertor circuit for generating anAC output and a contactor circuit for supplying a power output in orderto re-synchronise with the grid and begin exporting power to the grid sothat the turbogenerator speed is controlled in the normal way.

According to another embodiment, a method for controlling aturbogenerator system during a grid disturbance comprises: in responseto a grid disturbance detecting a condition of the turbogeneratoremploying a first sensing device and detecting a condition of the gridemploying a second sensing device, responsive to an output from thefirst sensing device issuing a first brake control signal for switchingon a brake to prevent the turbogenerator from acceleratinguncontrollably and a second brake control signal for switching off thebrake to permit resumption of normal operation of the turbogenerator,and responsive to an output from the second sensing device whichindicates a cessation of the grid disturbance activating a convertorcircuit for generating an AC output and a contactor circuit forsupplying a power output in order to re-synchronise with the grid andbegin exporting power to the grid so that the turbogenerator speed iscontrolled in the normal way.

The invention will be described further, by way of example, withreference to the accompanying drawings in which:

FIG. 1 is a schematic diagram of a turbogenerator system incorporatingthe invention;

FIG. 2 is a block circuit diagram of a power electronics unit within theturbogenerator system;

FIG. 3 is a schematic diagram of a braking arrangement and a monitoringand control system within the turbogenerator system;

FIG. 4 is a circuit diagram showing details of the braking arrangementfor the turbogenerator according to the present invention;

FIG. 5 is a flow chart illustrating operation of the monitoring andcontrol system of FIG. 3;

FIG. 6 is a further flow chart illustrating the operation of the systemof FIG. 3;

FIG. 7 is a signal diagram representing operation of a conventionalturbogenerator system; and

FIG. 8 is a signal diagram representing operation of a turbogeneratorsystem according to the invention.

Referring to the drawings, FIG. 1 shows a turbogenerator system 10adapted to be fitted to a diesel engine 12, in order to improve the fuelefficiency of the diesel engine 12 by recovering waste energy from theexhaust gases of the engine. The diesel engine 12 takes in fuel and airon lines 14 and 16, respectively, and supplies power for a maingenerator 18, whose output is connected via an overall output line 50 tothe local electrical grid network. The turbogenerator system 10comprises a turbogenerator 20 combining a turbine 24 and a high speedelectrical generator 26, wherein the turbine 24 is arranged to be drivenby the exhaust gases from the engine 12. Thereafter, the exhaust gasesare expelled by way of line 60. The output of the turbogenerator 20 isapplied by way of a line 20 a to a power electronics unit 22 forconversion into grid quality power for output. As shown in FIG. 1, theturbogenerator system 10 and the main generator 18 are effectivelyconnected in parallel for supplying an output to the local electricalgrid.

The speed of the turbogenerator 20 is controlled by adjusting the loadon the high speed electrical generator 26 by means of the powerelectronics unit 22, which exports variable amounts of power to thelocal electrical grid to hold the speed of the turbogenerator 20 at apredetermined value. The power electronics unit 22 includes a speedcontrol unit 28 able to detect a speed signal representing the speed ofthe turbogenerator 20 received therein and to compare the actual speedwith an internally preset speed set point to produce a speed errorsignal. The speed control unit 28 has programmed therein a speed controlalgorithm, which uses the speed error signal to generate a currentdemand signal for controlling the speed of the turbogenerator 20. Thespeed control unit 28 is a conventional proportional/integralcontroller, which calculates a speed error, and then proportional andintegral gain values, and which then applies these gain values tocalculate a current demand. Since the unit is conventional, furtherdescription is not given.

Referring to FIG. 2, the power electronics unit 22 comprises an AC/DCcircuit 30, which receives on input line 20 a a high frequency AC inputsupplied from the turbogenerator 20 and rectifies the same to produce asoutput a DC voltage that has not had any control applied to it.Connected to the output of the AC/DC rectifier circuit 30 is a DC/DCconverter 32 acting as a boost converter. The boost converter 32receives the rectified DC voltage from the AC/DC rectifier circuit 30 asinput, and produces as output a boosted and regulated DC link voltage,which is typically of the order of 700 volts. For this purpose, theboost converter 32 conventionally comprises an inductor and an IGBTswitch (not shown).

The DC link voltage is applied to a PWM inverter, comprising a convertercircuit 34 and a contactor circuit 36, which produces a three-phase ACoutput for supply to the local electrical grid. Typically, the convertercircuit 34 comprises three parallel DC/AC converters 34 a, 34 b, 34 c,each programmed to produce a sine wave output. The converters 34 providethree switching channels connected across the voltage bus and eachincluding an upper and a lower switching device in the form of an IGBTswitch and an anti-parallel diode (not shown). The output voltagesgenerated by the converters 34 generally contain a large content ofhigh-frequency ripple associated with the switching action of the IGBTswitches in the converters 34. Therefore, an EMC or three-phaseseries-inductance/parallel-capacitance (L-C) output filter 38 isconnected to the output of the contactor 36 to remove the majority ofthe high-frequency ripple.

In addition, the boost converter 32 includes the speed control unit 28,and the current demand signal from the speed control unit 28 is passedto the converters 34 a, 34 b, 24 c of the PWM inverter 34, 36, which isset up to act as a current source, for controlling the speed of theturbogenerator 20.

According to the invention, a braking arrangement is provided for thehigh speed electrical generator 26 of the turbogenerator 20. The brakingarrangement comprises a brake controller 40 connected to a brake circuit42, shown in greater detail in FIGS. 3 and 4. As shown, the brakecontroller 40 comprises a rectifier circuit 44, which receives the ACoutput from the turbogenerator 20 and rectifies and supplies such outputby way of a solid state switch 46, such as an IGBT, to the brake circuit42. The switch 46 is under the control of a monitor circuit 48 includedin the DC/DC converter 32 and serves to switch the brake circuit 42 inand out of operation for the purpose of controlling the speed of thehigh speed electrical generator 26, as explained below.

Referring to FIG. 4, the high speed electrical generator 26 of theturbogenerator 20 is shown as a six-phase generator supplying an outputto the rectifier circuit 44, shown as a six-phase rectifier. The brakingarrangement comprises a parallel connection of three resistors 52, whichcan be switched in and out of connection with the generator 26 under thecontrol of the IGBT switch 46 to apply a load to, or remove the samefrom, the generator 26 for respectively braking the turbogenerator 20 orallowing it to accelerate. The ohmic value of the resistors is fixed atthe time the system is manufactured, dependent on the application. Thevalue is preferably selected to limit the voltage to about 800V at themaximum expected power, but anywhere between 750V and 850V would also befeasible.

The monitor circuit 48 is provided for controlling the operation of theswitch 46, as shown in FIG. 3.

In normal operation of the generator 26, the switch 46 is maintained inthe open state and power is supplied to the turbogenerator 20 from thepower electronics unit 22 of FIG. 1. However, when an event occursnecessitating braking of the turbogenerator 20, the switch 46 isactivated in order to connect the resistors 52 to apply a load to thegenerator 26 and brake the turbogenerator 20.

The monitor circuit 48 comprises a turbogenerator detector 74 arrangedto monitor the state of the turbogenerator 20. The turbogeneratordetector 74 includes the speed control unit 28 and based on a simplealgorithm in the speed control unit 28 is responsive to the outputs froma pair of sensors respectively (not shown) for detecting by means of thespeed control unit 28 the speed of the turbogenerator 20 and the voltageoutput by the turbogenerator 20, and thence for generating detectionsignals accordingly. In the event that the turbogenerator 20 isoperating normally, these sensors will each generate a “normaloperation” signal for supply to the turbogenerator detector 74, and theturbogenerator detector 74 will output a “high” signal to maintain theswitch 46 in an open condition. On the other hand, if one of the sensorsdetects an abnormal state in the turbogenerator 20, for example that thespeed or voltage output exceed a predetermined threshold, such sensorwill provide an “abnormal operation” signal to the turbogeneratordetector 74, which will in turn output a “low” signal that will closethe switch 46. As a result, the resistors 52 will be brought intocircuit to apply a load to the generator 26 and brake the same.

The monitor circuit 48 further includes a fault detector 76 arranged todetect predetermined internal or external events that will affect theoperation of the turbogenerator 20 and to issue a signal to shut downthe engine 12 in response to such events. For this purpose, the faultdetector 76 is responsive to a series of sensors (not shown) fordetecting such events. For example, one of these sensors may be arrangedto detect failures in the electrical supply. Alternatively, or inaddition, one of the sensors might be a pressure transmitting devicearranged to detect impending engine compressor surge, or anaccelerometer arranged to detect excessive vibration. Otherpossibilities for the sensors include sensors for the detection ofinternal currents or voltages of the turbogenerator system, thedetection of the temperature of a heat sink of the turbogenerator, thedetection of software errors, the detection of loss of communicationsbetween the various circuit elements 32, 34, or the detection of failureof an internal memory of the electrical processing system for theturbogenerator 20.

Usually, the sensors associated with the fault detector 76 will generatea “normal” signal, and the monitor circuit 48 will provide a “high”output signal maintaining the engine 12 in a normal operating state.However, in response to the detection of an abnormal condition, such asone of those listed above, the associated sensor will generate acorresponding “abnormal” signal, and the fault detector 76 of themonitor circuit 48 will in turn will issue a “low” output signal fortriggering a set of events to shut down the engine 12.

Thus, as described with reference to FIG. 3, the monitoring circuit 48in the DC/DC converter 32 includes the turbogenerator detector 74arranged to receive electrical signals from the associated sensorsrepresenting the turbogenerator speed and voltage respectively and tosupply a brake signal as output in dependence upon the conditions. Themonitoring circuit 48 also includes the fault detector 76 receivingelectrical output signals from associated sensors and in responsesupplying as necessary an output in the form of a shutdown signal forthe engine 12. Further, the monitoring system 48 includes a grid monitor78 arranged to receive signals representing grid frequency, grid voltageand grid current, from additional sensors checking the state of theelectrical grid. The grid monitor 78 is arranged to supply a disconnectcontrol signal or a reconnect control signal in certain circumstances asdescribed below. Such signal is supplied to a control unit 80 arrangedto control the state of the converters 32, 34 and the contactor 36 inthe power electronics unit 22, as described below.

In normal operation, all of the sensor signals are monitored by themonitoring circuit 48, as shown in step 90 in FIG. 5. An on-goingprocess of checking for a fault takes place in step 92, and assumingthat no fault has arisen, the monitoring circuit 48 does not react andnormal operation of the turbogenerator system 10 continues unchecked.Only in the event that a fault arises does the system move from step 92to step 94, which signals the initiation of the process according to theinvention.

More especially, when a fault is detected within the monitoring circuit48 in step 94, then the power electronics unit 22 inspects the outputsof all the sensors associated with the two detectors 74, 76 and the gridmonitor 78 and establishes whether or not the fault is due to a griddisturbance. If the fault is due to a failure of the grid, then the gridmonitor 78 issues a grid disturbance signal to the control unit 80,which responds by shutting down the power electronics unit 22 bydisabling the converters 32 and 34 and temporarily placing them in astandby condition, and by opening the contactor 36. As a result, theturbogenerator 20 will no longer be loaded by the power electronicssystem 22 and will begin to accelerate. Accordingly, the speed of theturbogenerator 20, and the voltage output, will soon exceed apredetermined threshold, which will be detected by the associatedsensors. The outputs from the sensors will be received by theturbogenerator detector 74, which will generate an output to switch onthe brake 46. The monitoring system 48 will move to step 98.

On the other hand, an inspection of the sensors by the fault detector 76may show that the fault lies elsewhere and is not due to a griddisturbance. In this instance, the monitoring system 48 moves from step96 to step 100, and sends out a shutdown command to shut down the engine12 immediately. This ensures that the engine is shut down as aprecaution, in the event of faults, such as overheating, that may leadto internal damage and expense. The engine 12 typically has inputs thatcan accept signals from external devices, and one such input (not shown)is configured to accept a shutdown signal from the monitoring system 48.Receipt of this signal by the engine 12 will initiate an engine shutdown sequence. The shutdown signal is also applied via the control unit80 to the converters 32 and 34 to disable the same and shut down thepower electronics unit 22, thus leading to acceleration of theturbogenerator 20. This will then be detected by the monitor circuit 74,which will operate the brake 42. At the same time, therefore, thebraking arrangement for the turbogenerator 20 will be brought intoaction, due to acceleration of the turbogenerator 20, to stop theturbogenerator 20 as quickly as possible.

The process that is initiated with step 98 will now be described. Atthis point, the monitoring system 48 is monitoring the speed and voltageof the turbogenerator 20, employing the turbogenerator detector 74, aswell as the frequency and voltage of the grid, employing the gridmonitor 78.

The effect of the grid fault that has occurred is to remove the load onthe turbogenerator 20, which immediately begins accelerating. As soon asthe turbogenerator detector 74 detects that the speed of theturbogenerator 20 has exceeded a predetermined hysteresis threshold, orthat the turbogenerator voltage output has exceeded a predeterminedhysteresis threshold, the monitoring system 48 signals the brakecontroller 40 and activates the switch 46 to switch on the brake 42.Accordingly, the turbogenerator 20 decelerates, and its speed andvoltage output drop. When these drop below further hysteresisthresholds, the monitoring system 48 again detects the situation andthis time sends a signal to the brake controller 40 to switch off theswitch 46 and the brake 42. The turbogenerator 20 begins acceleratingagain. These steps may be repeated a number of times in a cycle duringthe course of step 98 over a waiting period to be described below withreference to FIG. 6.

Whilst the turbogenerator speed and voltage are being monitored by theturbogenerator detector 74, the grid frequency and voltage are beingmonitored by the grid monitor 78, in order to detect the state of thegrid and the moment when the grid disturbance ceases and the gridreturns to a healthy condition. As described above the DC/DC convertercircuit 32 and the DC/AC converters 34 are temporarily disabled at thistime. However, during this time, the boost converter, i.e. the DC/DCcircuit 32 and the DC/AC converters 34, is held in readiness to startagain, as shown in step 102 in FIG. 6. When the grid monitor 78 confirmsin step 104 that the grid frequency and voltage have risen to apredetermined level and are once again within the normal specification,the monitoring system 48 issues a grid healthy signal. At the same time,in step 106, the monitoring system 48 acting via the control unit 80activates the inverter circuit, in the form of the DC/AC converters 34and the contactor 36, employing the grid voltage as a reference signalto recommence generation of a sine wave output. Once the converters 34are synchronised to the grid, as described above, a command is sent toclose the contactor 36. The contactor 36 is thereby closed in step 108to ramp up the power output of the power electronics unit 22 to itsnormal state.

FIGS. 7 and 8 are signal diagrams showing the various signals outputrespectively in the case of the prior art and in the case of the presentinvention, by way of comparison. FIG. 7 pertains to the prior art, andwill be described first.

As shown in the first portion of FIG. 7, during normal operation thespeed of the engine 12 and the power of the generator 18 remain steady,as also does the speed of the turbogenerator 20. At time T1, a faultdevelops that is detected in the power electronics unit 22, so that theload on the turbogenerator 20 is removed and the turbogenerator 20begins to accelerate. At time T2, the turbogenerator speed and voltageoutput both reach a respective predetermined hysteresis threshold, andthe brake 46 is switched on.

Furthermore, in the conventional arrangement, responsive to thecontinuance of the fault for more than a brief preset period, a shutdownrequest is issued at time T3, causing the power output by the generator18 to ramp down and also reducing the power to the turbogenerator 20. Attime T4, the speed of the turbogenerator 20, and likewise its voltageoutput, drops below a hysteresis threshold, and this is detected by theturbogenerator detector 74. Accordingly, the brake 46 is switched offand the turbogenerator 20 begins accelerating again. This cycle ofalternately switching on and off the brake 46 is repeated until time T5,when the engine 12 is no longer producing enough exhaust gas to powerthe turbogenerator 20. At this point, the turbogenerator 10 shuts down.

Turning now to FIG. 8, which shows the situation according to thepresent invention, the normal situation is the same, until time T1 whena fault is detected in the power electronics unit 22, at which point theloss of the load due to the grid is noted. The turbogenerator 20 startsto accelerate, and at time T2, the speed of the turbogenerator 20, andlikewise the voltage output, exceeds a predetermined hysteresisthreshold, and the brake 46 is switched on. The cycle of switching thebrake 46 on and off occurs a number of times, dependent on the speedand/or voltage output of the turbogenerator 20, as before. By contrastwith the prior art, the power electronics unit 22 does not disconnectthe engine 12 from the grid and takes no immediate steps to do so. Attime T3, the cycle of switching the brake 46 on and off continues.

Furthermore, again by contrast with the prior art, at time T4, themonitoring system 48 notes from the grid monitor 78 that the griddisturbance has been removed and that the grid has returned to normal,thereby removing the fault detected in the power electronics unit 22.The monitoring system 48 then issues a grid healthy signal in order tore-activate the inverter circuit, comprising the DC/DC converters 34 andthe contactor 36, to generate a sine wave output again and to close thecontactor 36 to begin ramping up the power output from theturbogenerator 22, and to reconnect to the grid at time T5. The processof re-activating the inverter circuit is described above.

The above described embodiments use an electrically resistive (Ohmic)brake. Such a brake uses electrical resistance rather than mechanicalfriction in order to slow down the turbogenerator. As such, the brakecircuit does not suffer from wear and tear in the way of the mechanicalbrake. Furthermore, it may be faster acting, and may allow energy to bedissipated in a more controlled manner than a mechanical brake.Nonetheless, it will be appreciated that the present invention need notbe limited to an electrically resistive brake, and may instead make useof a physical brake which is responsive to the first and second brakecontrol signals to apply and remove respectively a braking force tocontrol the speed of the turbogenerator. Such mechanical brakes are wellknown, and are not described further herein.

As explained above, the present invention is applicable in any situationin which the power electronics circuit is unable to export enough powerto form a sufficient load on the turbogenerator to prevent theturbogenerator from accelerating uncontrollably. The above example isthat of where this is caused by a grid disturbance, which leads to thedeactivation of the power electronics circuit, but the problem may ariseunder other circumstances as outlined previously. Where the problem isnot due to a disturbance in the grid, then the power electronics circuitmay not need to be deactivated, but instead may be used in combinationwith the brake to regulate the speed of the turbogenerator. It will beunderstood that the brake is activated in dependence on the condition(e.g. speed or output voltage) of the turbogenerator, which will exceedallowable bounds only in the event that the power electronics circuit isunable to regulate the turbogenerator speed unaided. There is norequirement to monitor the performance of the power electronics circuitdirectly. It will also be appreciated that, even where the brake isintended to regulate the turbogenerator speed in the event of adisturbance in the grid, then other techniques (i.e. in the alternativeto deactivation of the power electronics circuit) could be used toisolate the power electronics circuit from the grid.

1. Apparatus for controlling a turbogenerator system when a powerelectronics circuit of the turbogenerator system is unable to provide asufficient load on the turbogenerator to prevent the turbogenerator fromaccelerating uncontrollably, comprising: a monitor including a firstsensing device operable to detect a condition of the turbogenerator, anda brake controller responsive to a turbogenerator detection output fromthe first sensing device to issue a first brake control signal foroperating a brake to prevent the turbogenerator from acceleratinguncontrollably and a second brake control signal for operating the braketo permit resumption of normal operation of the turbogenerator. 2.Apparatus according to claim 1, wherein the power electronics circuit isdeactivated during a grid disturbance, comprising a second sensingdevice operable to detect a condition of the grid, and a trigger circuitresponsive to a grid detection output from the second sensing deviceindicating a cessation of the grid disturbance to reactivate the powerelectronics unit and begin exporting power to the grid.
 3. Apparatusaccording to claim 1 or claim 2, wherein the brake is a physical brakeresponsive to the first and second brake control signals to apply andremove respectively a braking force to control the speed of theturbogenerator.
 4. Apparatus according to claim 1 or claim 2, whereinthe brake is a brake circuit.
 5. Apparatus according to claim 2 whereinthe trigger device is responsive to a grid detection output from thesecond sensing device indicating a grid disturbance to de-activate thepower electronics unit.
 6. Apparatus according to claim 4 wherein thebrake controller comprises a rectifier circuit arranged to rectify an ACoutput from the turbogenerator and to supply the rectified output to thebrake circuit, and a switching device operable by the output from thefirst sensing device for respectively issuing the first and second brakecontrol signals for controlling the supply of the rectified output tothe brake circuit.
 7. Apparatus according to any preceding claim whereinthe first sensing device is arranged to monitor at least one of thespeed and the output voltage of the turbogenerator with respect topredetermined upper and lower thresholds respectively for generating thesaid turbogenerator detection output.
 8. Apparatus according to claim 7wherein the upper and lower thresholds are hysteresis thresholds, andwherein the first sensing device is responsive to said hysteresisthresholds to generate the turbogenerator detection output to cause thebrake controller cyclically to generate the first and second brakecontrol signals.
 9. Apparatus according to claim 2 wherein the powerelectronics unit comprises a convertor circuit for generating an ACoutput and a contactor circuit for supplying a power output insynchronisation with the grid, and wherein the trigger device isarranged to respond to the grid detection output from the second sensingdevice indicating a cessation of the grid disturbance by activating theconvertor circuit and the contactor circuit and by initiating are-synchronisation with the grid.
 10. Apparatus according to claim 2wherein the second sensing device is arranged to monitor at least one ofgrid frequency, grid voltage and grid current with respect topredetermined upper and lower thresholds for generating the saiddetection output.
 11. A method for controlling a turbogenerator systemwhen a power electronics circuit of the turbogenerator system is unableto provide a sufficient load on the turbogenerator to prevent theturbogenerator from accelerating uncontrollably, said method comprising:detecting a condition of the turbogenerator employing a first sensingdevice, responsive to an output from the first sensing device issuing afirst brake control signal for switching on a brake to prevent theturbogenerator from accelerating uncontrollably and a second brakecontrol signal for switching off the brake to permit resumption ofnormal operation of the turbogenerator.
 12. A method according to claim11, wherein the power electronics circuit is deactivated during a griddisturbance, the method comprising detecting a condition of the gridemploying a second sensing device, and responsive to a grid detectionoutput from the second sensing device which indicates a cessation of thegrid disturbance activating the power electronics unit and beginning toexport power to the grid.
 13. A method according to claim 11 or claim12, wherein the brake is a physical brake responsive to the first andsecond brake control signals to apply and remove respectively a brakingforce to control the speed of the turbogenerator.
 14. A method accordingto claim 11 or claim 12, wherein the brake is a brake circuit.
 15. Amethod according to claim 12 comprising de-activating the powerelectronics unit responsive to a further grid detection output from thesecond sensing device indicating a grid disturbance.
 16. A methodaccording to claim 14 further comprising operating the brake by means ofan output from a rectifier circuit arranged to rectify an AC output fromthe turbogenerator, and employing a switching device for respectivelyissuing the first and second brake control signals and controlling thesupply of the rectified output to a brake circuit.
 17. A methodaccording to any of claims 11 to 16 comprising monitoring at least oneof the speed and the output voltage of the turbogenerator with respectto predetermined upper and lower thresholds respectively for generatingthe said turbogenerator detection output.
 18. A method according toclaim 17 wherein the upper and lower thresholds are hysteresisthresholds, and comprising responding to said hysteresis thresholds togenerate the turbogenerator detection output cyclically for producingthe first and second brake control signals.
 19. A method according toany of claim 12 wherein the power electronics unit comprises a convertorcircuit for generating an AC output and a contactor circuit forsupplying a power output in synchronisation with the grid, and furthercomprising responding to the grid detection output from the secondsensing device indicating a cessation of the grid disturbance byactivating the convertor circuit and the contactor circuit and byinitiating a re-synchronisation with the grid.
 20. A method according toclaim 12 comprising monitoring at least one of grid frequency, gridvoltage and grid current with respect to predetermined upper and lowerthresholds for generating the said grid detection output.